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Effects of dissolved ions and natural organic matter on electrocoagulation of As(III) in groundwater
Accepted Manuscript Title: Effects of Dissolved Ions and Natural Organic Matter on Electrocoagulation of As(III) in Groundwater Author: Han Jo You Ihn Sup Han PII: DOI: Reference:
S2213-3437(15)30123-8 http://dx.doi.org/doi:10.1016/j.jece.2015.12.034 JECE 916
To appear in: Received date: Revised date: Accepted date:
27-8-2015 2-12-2015 29-12-2015
Please cite this article as: Han Jo You, Ihn Sup Han, Effects of Dissolved Ions and Natural Organic Matter on Electrocoagulation of As(III) in Groundwater, Journal of Environmental Chemical Engineering http://dx.doi.org/10.1016/j.jece.2015.12.034 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effects of Dissolved Ions and Natural Organic Matter on Electrocoagulation of As(III) in Groundwater Han Jo You, Ihn Sup Han*
Address : School of Environmental Engineering, University of Seoul, Siripdae-gil 163, Dongdaemun-gu, Seoul, Korea
Telephone : +82264905478
Fax : +82264902859
E-mail address: [email protected] (H.J. You)
*Corresponding author. E-mail address : [email protected] (I.S. Han) Telephone : +82264905478
Fax : +82264902859
Highlights The use of an iron electrode resulted in a higher removal efficiency than an aluminum electrode. The fastest removal efficiency was observed at pH 7. When the magnesium concentration was less than 10 mg/L, the arsenic removal was accelerated. The presence of phosphate ions and humic acid decreased the arsenic removal rate.
Abstract Electrocoagulation is an outstanding technique to remove pollutants. When current is applied to electrodes, various amorphous iron hydroxides form complexes. Those complexes with high absorption capacity such as arsenic and heavy metals are removed through the process. In this study, the effects of various electrocoagulation conditions, such as the type of electrode, current, and pH, on the removal efficiency of arsenic were investigated. The removal efficiency varied significantly depending on the type of electrode, as an iron electrode showed superior arsenic removal as compared to an aluminum electrode. As the current increased, the removal rate of arsenic has increased. The validity of the method was examined by calculating the metal elution during electrocoagulation using Faraday’s law and comparing it to the actual elution amount. Notably, amongst the various pH conditions pH 7 generated the fastest removal rate. The effects of dissolved ions on arsenic removal were also examined. When the magnesium concentration was less than 10 mg/L, the initial arsenic removal speed increased. No effects were observed when the concentration of sulfate ions was low (1 mg/L, 10 mg/L). When the concentration of sulfate ions was high (100 mg/L), the arsenic removal rate decreased. In addition, the presence of phosphate ions and humic acid (HA) are reversely correlated with arsenic removal rate.
Keywords: Arsenic, Electrocoagulation, Iron hydroxide, Ion, Humic acid
1. Introduction Arsenic is an artificial pollutant prevalent in leachates in mine areas and agricultural chemicals (e.g., agricultural pesticides and herbicides). Arsenic-containing minerals dissolve into water source and cause natural pollution [1]. Most of the arsenic pollution is natural pollution. Arsenic is abundant in earth’s crust, and thus environmental arsenic pollution is observed worldwide. Arsenic has adverse effects on humans and can cause various skin diseases with long-term exposure; in severe cases, it can cause skin cancer and death [2,3]. World Health Organization (WHO) have established an allowable level of arsenic in drinking water to be 0.01 mg/L, many countries including South Korea abide the standard [3,4]. Globally, numerous studies regarding the removal of arsenic from water have been performed [1]. The main technologies used for arsenic removal include precipitation technologies (coagulation, Fe/Mn oxidation, lime softening), membrane technologies (reverse osmosis, electrodialysis), ion exchange, and adsorption (activated alumina, granular ferric hydroxide, granular titanium dioxide) [5,6]. In particular, electrocoagulation is effective for removal of As(III) [7-9]. Many researchers have found that electrocoagulation is effective for removal of pollutants in water as well as removal of arsenic [10-21]. Equations 1-4 illustrate the reaction that occurs when an iron electrode is used for electrocoagulation [22]: Anode: 4Fe(s) → 4Fe2+(aq) + 8e-
(1)
4Fe2+(aq) + 10H2O(l) + O2(g) → 4Fe(OH)3(s) + 8H+(aq)
(2)
Cathode: 8H+(aq) +8e- → 4H2(g)
(3)
Overall: 4Fe2+(aq) + 10H2O(l) + O2(g) → 4Fe(OH)3(s) + 4H2(g)
(4)
In groundwater, arsenic primarily exists in the trivalent form; many ions and organic matters also exist in groundwater [4]. Metal ions and organic matter are expected to have a large effect on the removal of arsenic from groundwater through electrocoagulation [23]. In this study, effects of magnesium and sulfate as well as phosphate and natural organic matter on the removal of arsenic were investigated. Existing researches have focused only on the efficiency of arsenic removal. Studies about residual iron concentration as secondary pollution are insufficient. High iron ion concentration is not as toxic as arsenic but it has chronic significance on health and environment. The correlation between ion profile of natural water bath and residual iron ion is a key to future research on water resource and environment protection. The study have investigated ion concentration levels within the quality standard of drinking water.
2. Materials and methods 2.1 Materials Samples were prepared using artificial raw water. Basic artificial raw water was made with ultrapure water, As(III), NaCl, HCl, and NaOH. Ultrapure water was the primary source for the artificial raw water and all solutions. To prepare As(III), a stock solution was made using NaAsO2, and the arsenic concentration of the artificial raw water was set to 1 mg/L. NaCl (about 500 μS/cm) was injected at a total concentration of 0.004 M to provide conductivity for the electrocoagulation process. About 0.1 M NaOH and HCl solutions were used to adjust the pH. The pH was determined using a pH meter (238-180 model, Istek, Korea). Only As(III), NaCl, HCl, and NaOH were injected into the raw water. When the effects of ions were examined, each ion species was injected individually, in order to minimize the effects of other ions. Magnesium, phosphate, sulfate, and humic acid stock solutions were made from MgCl2·6H2O, Na2HPO4, Na2O(17.019.0%)+SiO2(35.0-38.0%), Na2SO4, and humic acid, respectively.
2.2 Devices and experimental methods Fig. 1 shows the electrocoagulation and filtering processes used in this study. The size of the reactor was W190 mm * L120 mm * H250 mm. The reactor had a volume of 4 L, and was made of acrylic plates. The size of the electrode plate was W85 mm * L3 mm * H260 mm, and the plates were installed based on a bipolar serial connection (BP-S), where they were fixed at 10 mm intervals so that the wide faces could face each other. Before each experiment, the electrode plate was immersed in 1 M HCl for 10 min, and was then polished using sandpaper. Scale and rust on the iron electrode were removed as mentioned above, and it was then used for the experiment after cleaning with distilled water, which increased the reproducibility of the experiments. A constant current was supplied using a D.C. power supply. The interior of the reactor was uniformly agitated at 60 rpm. During the electrocoagulation process, samples were collected at specified times. The samples were filtered through a 0.45 μm membrane filter (Advantec, mixed cellulose ester) and were subsequently analyzed. Initial arsenic concentration of raw water and accurate sampling time are crucial factors for reducing errors.
2.3 Analytical methods The samples were analyzed using the following machinery and methods. Mg and Fe contents were measured using Inductively Coupled Plasma (ICP, ICPE-9000, Shimadzu, Japan). It was difficult to measure low concentrations of arsenic using ICP alone; therefore, As was analyzed using Inductively Coupled Plasma - Hydride Vapor Generator (ICP-HVG, Shimadzu, Japan). Ferrous iron was analyzed using a UV/VIS spectrophotometer (Evolution 60S, Thermo, USA) based on the 3500-Fe Phenanthroline Method of the Standard Methods [24]. The change in PO43- content was measured using Ion Chromatography (IC, ICS-900, Dionex, USA). To examine the removal efficiency of humic acid, the Dissolved Organic Carbon (DOC) was measured using a Total Organic Carbon Analyzer (TOC Analyzer, TOC-V, Shimadzu, Japan), and the adsorption at 254 nm was measured using a UV/VIS spectrophotometer. A Zetasizer (Nano ZS, Malvern, UK) was used to measure the zeta potential. The electrocoagulation by-
product was precipitated and dried, and was analyzed using X-Ray Diffraction (XRD, D8 advance Sol-X, Bruker, Germany).. In order to reduce sample errors, maintaining standard calibration curves of the equipment played a crucial role to keep consistent and accurate results.
3. Results and discussion 3.1 Comparison of aluminum and iron electrodes Iron or aluminum electrodes are used in most cases for electrocoagulation. Iron electrodes generally are more corrosive than aluminum electrodes; when an iron electrode is used, a red residue is formed. Therefore, for materials that show similar removal efficiencies, aluminum electrodes are preferred [25]. Iron and aluminum electrodes were evaluated in the removal of arsenic. Four electrode plates with the same area were used for both electrodes, and the removal efficiencies were determined at various currents. A distinct difference between the electrodes was observed, as shown in Fig. 2. With the iron electrode, the removal efficiency was 99% within 10 min at 0.2 A; at 0.1 A, a removal efficiency of 99% was achieved 15 min. At 0.05 A, the removal efficiency was higher than 99% within 25 min. However, with the aluminum electrode, the arsenic removal efficiency was less than 50% until 30 min at 0.2 A and 0.1 A. As the current increased, the elution of the metal electrode increased, and thus the formation of metal hydroxides increased. Presumably, the fast-forming metal hydroxide aids in the quick removal of arsenic. However, the aluminum electrode showed significantly lower arsenic removal than the iron electrode. This indicated that iron hydroxide binds better to arsenic than aluminum hydroxide [9]. In subsequent experiments, 0.1 A current was used instead of 0.2 A, because the fast removal of arsenic was not distinctly different in the presence of other ions.
3.2 Elution of iron In electrocoagulation, the amount of metal ions dissolved in water from an electrode plate is proportional to the reaction time and current. The current was fixed at 0.1 A at pH 7, and the electrical conductivity was adjusted using NaCl. First, the elution was compared by varying the number of plates (2, 3, and 4 plates). As the number of plates increased, the concentration of iron also increased. This was compared to the theoretical concentration using Faraday’s law [3,26]. When 2 iron plates were used, the coefficient of determination (R2) between the theoretical and experimental values was very high (0.9941), as shown in Fig. 3. The absolute average error was also very small (5.2395).
However, when 3 and 4 plates were used, the concentration of iron increased continuously. The results were compared by including a factor for the number of plates in Faraday’s law, as shown in equation 5: 1
(5)
where W is the mass of the dissolved metal (g), I is the applied current (A), t is the treatment time of the
electrocoagulation process (s), M is the molar mass of the anode metal (g/mol), z is the valence number of ions of the substance (zAl = 3, zFe = 2), F is Faraday's constant (96,485 C/mol), and n is the number of plates. The coefficients of determination (R2) between the theoretical and experimental values when the plate numbers were 3 and 4 were 0.9931 and 0.9974, respectively, as summarized in Table 1. The absolute average errors were small (3.7432 and 4.6096, respectively). As a result, when more than 2 plates were used, the metal elution was proportional to 1 less than the number of plates (n). When the experiment was performed at pH values of 5, 7, and 9 using 4 plates, the absolute average errors were within 5%. Therefore, the use of Faraday’s law was appropriate. However, at pH 3, the absolute average error was 36.2848%. The experimental value was significantly larger than the theoretical value because the dissolution rate of the positive electrode was accelerated under acidic conditions. At pH values less than 4, hydrogen evolution occurs, and it is called hydrogen evolution-type corrosion. At pH values between 4 and 10, the corrosion rate is nearly constant [27]. At approximately pH 7, the protective oxide film on the electrode surface is not stable, and thus the iron elution at the electrode is high [28]. At pH 3, the rate at which the pH increases is slow, and thus more elution of iron occurs until the pH increases above a certain value. Therefore, Faraday’s law could not be applied at pH 3.
3.3 Effect of initial pH on removal efficiency In the following experiment, 4 iron plates were used, and the effect of pH on arsenic removal was examined by adjusting the pH (3, 5, 7, and 9) using NaOH and HCl. At pH 7, the arsenic content reached 0.01 mg/L (i.e., the standard for drinking water) in the shortest amount of time, as shown in Fig. 4. During the initial 8 min of the 30-minute electrocoagulation process, the highest removal efficiency was observed at pH 5. Due to the fast oxidation of iron, arsenic was removed quickly. However, after 10 min, the best arsenic removal efficiency was observed at pH 7. At pH 7, after 10 min, fast-grown flocs were filtered by a membrane filter, and a higher removal efficiency was observed. At pH 9, the removal efficiency was lower than that at an initial pH of 5, but was similar to that at pH 7. After 5 min, the removal efficiency significantly decreased compared to those at pH values of 7 and 5. Although the removal rate decreased, the water quality standard of arsenic (0.01 mg/L) could be satisfied within 30 min at pH 9. However, at pH 3, the arsenic content could not reach 0.01 mg/L within 30 min. Fig. 5 shows the concentration of iron that remained in water after electrocoagulation and membrane filtering. At pH 7 and 9, when the arsenic concentration decreased below 0.01 mg/L, the iron concentration also decreased below 0.3 mg/L (i.e., the water quality standard of drinking water). During the initial stages, iron mostly existed as ferric iron, and iron hydroxide remained as small particles. The amount of remaining ferrous iron was much less than that of ferric iron. However, at pH 3 and 5, a high concentration of iron (more than 0.3 mg/L) was observed. Most of the remaining iron at pH 3 and 5 was ferrous iron, and it could not be oxidized to ferric iron and remained in the water. The concentration of dissolved iron was likely high because iron hydroxide could not form because the dissolution of iron hydroxide is high at low pH. Moreover, the iron dissolution increased as the pH decreased. Removal of arsenic absorbed by iron hydroxide through membrane filtration decreased at pH 5. Fig. 4 shows arsenic concentration fall in pH7 after 10min compared to that of pH5.
3.4 Effects of metal ions and humic acid Next, we examined the effects of dissolved ions on the removal of arsenic from groundwater via electrocoagulation. First, to investigate the effects of ion concentration, different concentrations of selected ions were injected at a current of 0.1 A, and the pH was adjusted to 7.
3.4.1. Effects of magnesium First, the effects of dissolved magnesium ions on the removal of arsenic from groundwater via electrocoagulation were evaluated. A concentration of 0.1 mg/L of magnesium had almost no effect on the arsenic removal (data not shown). When the concentrations of magnesium were 1 mg/L and 10 mg/L, the arsenic removal rate increased; this trend was more distinct when the concentration of magnesium was high (10 mg/L). However, the presence of 100 mg/L of magnesium decreased the arsenic removal rate. As shown in Fig. 7, more than 0.3 mg/L (i.e., water quality standard of drinking water) of iron remained when 10 mg/L of magnesium was injected, and the amount of iron continuously increased as time passed. In particular, when 100 mg/L of magnesium was added, the concentration of remaining iron was compared to the total amount of iron dissolved in water that was calculated using Faraday’s law. The concentration of iron remaining in water after 3 min was about 44% of the total amount of iron, and it was about 55% at 30 min, where it steadily increased as time passed. The remaining iron was identified as ferrous iron, while unoxidized iron was present in the filtered water. Because magnesium combines with hydroxide ions to produce Mg(OH)2 [29,30] (Eq. 7), the production of iron hydroxide decreased. Thus, ferrous iron in water could not be oxidized and remain in the water. The decrease in iron hydroxide content reduced the arsenic removal rate.
MgCl2 + 2OH- → Mg(OH)2 + Cl2
(7)
The change in magnesium content could were also examined using ICP. The decrease in magnesium was about 10% at all concentrations excluding 0.1 mg/L.
The solubility product of Mg(OH)2 ⇄ Mg2+ + 2OH- (Ksp at 25°C) is 9×10-12 [31]. It is a solid and it dissociates in water. Thus, more OH- was deficient, and this resulted in the decrease in iron hydroxide. Table 2 summarizes the zeta potential measurements. With 100 mg/L of magnesium, the zeta potential was positive at 3 min. Negatively charged colloids or anions were quickly removed as they approached 0 mV due to the dissolution of iron (i.e., cation). However, the arsenic removal was not efficient because of the reversed potential, due to the high concentration of magnesium.
Appropriate amounts of magnesium had a positive effect on arsenic removal, but high concentrations of magnesium decreased the arsenic removal rate due to the increase in the zeta potential and the inhibition of
coagulant formation. To prevent this, large amounts of magnesium should be removed first. Additionally, the pH should be increased, and the time should be adjusted to reduce the iron elution.
3.4.2. Effects of sulfate ions The effects of sulfate ions on arsenic removal were evaluated. Changes in the arsenic removal were examined by injecting 0.1, 1, 10, and 100 mg/L solutions of sulfate ions at pH 7. The results obtained in the presence of 0.1 mg/L sulfate ions are not shown in Fig. 8. With 1 and 10 mg/L sulfate ions, the results were similar to that without sulfate. However, at 100 mg/L, the arsenic removal rate decreased. In previous studies, it was reported that sulfate ions had almost no effect on arsenic removal [8,23]. In the present study, 10 mg/L of sulfate also had no effect on the arsenic removal rate. However, at high concentrations of sulfate (100 mg/L), the arsenic removal rate decreased. SO42- probably competed with arsenic for the adsorption site of iron hydroxide [32]. Also, considering that the concentration of iron initially increased and then decreased when the sulfate concentration was 100 mg/L (Fig. 8), the oxidation of iron was probably inhibited.
3.4.3 Effects of phosphate ions Phosphate ions have been reported to inhibit removal of arsenic. Arsenic and phosphorus are located in the same group of the periodic table, and have the same tetrahedral structure. Thus, they are removed by adsorption on the same adsorption site of iron hydroxide [8,33,34]. Phosphate ions are less abundant in groundwater than other ions. In this experiment, 0.1, 1, and 10 mg/L of phosphate ions were added, and the removal efficiency of arsenic as well as the concentration of dissolved iron were examined. As shown in Fig. 9, the arsenic removal rate was affected by the presence of 0.1 mg/L phosphate, unlike that with other ions. At about 8 min, 0 and 0.1 mg/L showed a C/C0 difference of about 0.2. The times required to reach the drinking water quality standard were not significantly different. However, with 0.1 mg/L phosphate, it took more time for the concentration of iron to decrease below the drinking water quality standard. As the concentration of phosphate ions increased, the removal rate decreased. When 10 mg/L of phosphate ions were injected, the arsenic removal efficiency was 70% within 30 min, and the drinking water quality standard could not be satisfied. With 10 mg/L of phosphate ions, the arsenic removal efficiency depending on the initial pH was examined by increasing the electrocoagulation process time to 60 min. The concentration of phosphate ions was measured using ion chromatography; phosphate ions were not detected in the sample collected at 5 min at 1 mg/L. The removal of phosphate ions was faster than that of arsenic. Furthermore, when the concentration of iron decreased below the standard value, it was nearly consistent until the water quality standard for arsenic was reached. This indicated that arsenic and phosphate ions have a competitive relationship, but phosphate ions are quickly removed because they react with iron faster than arsenic, and that the removal of arsenic is relatively more difficult.
Fe3+ + PO43- → FePO4
(8)
Phosphate is removed by the above mechanism (Eq. 8), and it could also be removed by adsorption on iron hydroxide [35,36]. Due to this removal mechanism, OH- produced during the electrocoagulation process remained in the water and did not form iron hydroxide, which increased the pH. This increase was compared
to that observed in other experiments. With 10 mg/L of magnesium (Fig. 6) and 0 mg/L of phosphate (Fig. 7), the pH values after 15 min were 6.67 and 7.18, respectively, but for 10 mg/L of phosphate, the pH increased to 9.52.
When 10 mg/L of phosphate ions were dissolved in water, the arsenic removal efficiency was low. Therefore, the process time was increased to 60 min. However, after 60 min, the removal efficiency was only about 90%, and the drinking water quality standard could not be satisfied.
3.4.4 Effects of humic acid The effects of humic acid on the removal of arsenic from groundwater were also evaluated. Humic acid concentrations of 1 mg/L, 5 mg/L, 10 mg/L, and 20 mg/L were assessed. Fig. 11 shows the removal efficiency of arsenic in the presence of humic acid. There was almost no difference in the arsenic removal rate or iron concentration upon addition of 1 mg/L humic acid as compared to that without humic acid. When the concentrations of humic acid were 5 mg/L, 10 mg/L, and 20 mg/L, the arsenic removal rate decreased as the concentration increased, and a significant amount of time was required to reach a concentration below the drinking water quality standard. The target water quality was obtained at 20 min for 10 mg/L, and at 25 min for 20 mg/L. Fig. 12 shows the concentration of remaining iron. The iron concentration increased as the humic acid concentration increased. This pattern was similar to that of other ions, but there was a slight difference. While the concentration of remaining iron ions increased during the initial stage of the process, the decrease in arsenic content was smaller than the case in which the inhibition of arsenic removal by ions was observed. When the concentration of remaining iron began to decrease, the arsenic and iron concentrations abruptly decreased. In particular, the difference was large when the ion concentration was high.
These results were attributed to the sweep coagulation of humic acid. Humic acid is comprised of hydrophobic and hydrophilic materials. In particular, the hydrophobic compounds constitute about 90% of the total material. In general, hydrophobic compounds are more easily coagulated [37]. Thus, the coagulation and formation of iron hydroxide flocs are improved; as a result, sweep coagulation occurs due to the decrease in the zeta potential of the floating colloidal material and the promotion of colloidal particle removal by trapping [38]. The removal of humic acid was measured using the adsorption at 254 nm and DOC. In previous study, it was mentioned that the increase in the adsorption at 254 nm during electrocoagulation using an iron electrode was due to production of colored iron hydroxide particles [39]. In this study, the UV adsorption at 254 nm also increased with the addition of 10 mg/L of humic acid. The adsorption at 254 nm increased to 3.6 times the initial value, then decreased to 0.05 after 15 min; DOC revealed a removal efficiency of 83.5%. This value was continuously maintained until 30 min. In addition, as mentioned previously, the concentrations of arsenic and iron decreased abruptly at 15 min, indicating that humic acid and arsenic were removed simultaneously.
Table 3 compares the cases in which there was humic acid in water when there was no humic acid. Organic acids such as humic acid have a negative charge in water, which usually inhibits coagulation and adsorption. In this experiment, humic acid exhibited a negative charge and lowered the coagulation and adsorption of arsenic [39].
3.5 XRD To confirm the formation of iron hydroxide, the particles produced after the electrocoagulation process were collected and dried, and were analyzed using X-ray Diffraction (XRD). Fig. 14 and Fig. 15 show the XRD of the products obtained after 60- and 30-minute electrocoagulation processes. Respectively, at pH 7 when the arsenic concentration was 1 mg/L. Iron existed as lepidocrocite (γ-FeO(OH)) and magnetite (Fe3O4) (Fig. 14). However, only lepidocrocite was observed after 30 min (Fig. 15), indicating that the type of iron that formed varied as time passed. Magnetite is an oxidized form of iron, and its formation over time is consistent with the electrocoagulation mechanism mentioned previously [8,40]. Fig. 16 shows the results of a 30-minute electrocoagulation process at pH 7 when 1 mg/L As(III) and 10 mg/L humic acid were present, only lepidocrocite was observed in this process. Lepidocrocite formation was observed under several sets of conditions (Figs. 14, 15, and 16), but the peaks intensity in the XRD spectra became sharper and clearer as time passed. In addition, when arsenic or humic acid were present, the peak intensity became weak.
Through XRD analysis, the electrocoagulation mechanism was determined to proceed as shown in equations 9-12.
Early stage: Fe + 3(OH) → Fe(OH)3 Fe(OH)3(s) → FeO(OH)(s) + H2O(l) (lepidocrocite)
(9) (10)
Later stage: 4Fe + 12(OH) → 4Fe(OH)3
(11)
4Fe(OH)3(s) → Fe(OH)2(s) + Fe3O4(s) + H2O(l) (magnetite) (12)
During electrocoagulation, arsenic is likely removed via adsorption and co-precipitation by Fe(OH)3 and lepidocrocite [26]; during the late stages of electrocoagulation, arsenic is removed by the formation of magnetite [40].
4. Conclusions In this study, the effects of various ions and humic acid on the removal of As(III) via electrocoagulation were investigated. The use of an iron electrode resulted in a higher removal efficiency than an aluminum electrode. The time required to reach the drinking water quality standard varied depending on the pH. The fastest removal efficiency was observed at pH 7, followed by pH 5, pH 9, and pH 3. Magnesium increased the arsenic removal rate. However, high magnesium concentrations inhibited arsenic removal, and abruptly increased the concentration of dissolved iron. A high concentration of sulfate or phosphate ions decreased the arsenic removal rate, and resulted in a substantial increase in the pH. In addition, humic acid also decreased the arsenic removal rate. XRD analysis indicated that iron was oxidized to lepidocrocite initially and subsequently oxidized to magnetite during the later stages of the process. These iron hydroxide and iron oxide species aided in the effective removal of arsenic.
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Figures and figure captions
Fig. 1. Electrocoagulation and membrane process.
100 0.2A Fe 0.1A Fe 0.05A Fe 0.1A Al 0.2A Al
Arsenic removal (%)
80 60 40 20 0 0
5
10
15
20
25
30
Time(min)
Fig. 2. Arsenic removal as a function of current on iron and aluminum plates.
Experimental concentration of Fe (mg/L)
7 6 5 4 3 2 1 0 0
1
2
3
4
5
6
7
Theoretical concentration of Fe (mg/L)
Fig. 3. Comparison between theoretical and experimental iron concentrations using 2 iron plates at pH 7.
1 pH 3 0.8
pH 5 pH 7
0.6 C/C0
pH 9
0.4 0.2 0 0
5
10
15
20
25
30
Time(min)
Fig. 4. Arsenic removal as a function of initial pH after electrocoagulation and membrane process at 0.1 A.
10
Fe concentration(mg/L)
pH 3 8
pH 5 pH 7
6
pH 9
4 2 0 0
5
10
15
20
25
30
Time(min)
Fig. 5. Residual Fe concentration as a function of initial pH after electrocoagulation and membrane process.
1 Magnesium ion 0 mg/L
0.8
Magnesium ion 1 mg/L 0.6 C/C0
Magnesium ion 10 mg/L Magnesium ion 100 mg/L
0.4 0.2 0 0
5
10
15
20
25
30
Time(min)
Fig. 6. Effects of magnesium concentration on arsenic removal.
Fe Concentration(mg/L)
10.000 8.000 6.000 Magnesium ion 0 mg/L Magnesium ion 1 mg/L
4.000
Magnesium ion 10 mg/L Magnesium ion 100 mg/L
2.000 0.000 0
5
10
15
20
25
30
Time(min)
Fig. 7. Residual Fe concentration as a function of magnesium ion concentration.
1 0.8 Sulfate 0 mg/L 0.6 C/C0
Sulfate 1 mg/L Sulfate 10 mg/L
0.4
Sulfate 100 mg/L
0.2 0 0
5
10
15
20
25
30
Time(min)
Fig. 8. Effect of sulfate concentration on arsenic removal.
1
phosphate 0 mg/L phosphate 0.1 mg/L phophpate 1 mg/L phosphate 10 mg/L
0.8
C/C0
0.6 0.4 0.2 0 0
5
10
15
20
25
30
Time(min)
Fig. 9. Effect of phosphate concentration on arsenic removal.
12
Fe Concentrion(mg/L)
10 8 6 phosphate 0 mg/L phosphate 0.1 mg/L phophpate 1 mg/L phosphate 10 mg/L
4 2 0 0
5
10
15
20
25
30
Time(min)
Fig. 10. Residual Fe concentration as a function of phosphate concentration.
1 humic acid 0 mg/L
0.8
humic acid 5 mg/L 0.6 C/C0
humic acid 10 mg/L humic acid 20 mg/L
0.4 0.2 0 0
5
10
15
20
25
30
Time(min)
Fig. 11. Effect of humic acid concentration on arsenic removal.
12
Fe Concentration(mg/L)
10 humic acid 0 mg/L 8
humic acid 5 mg/L humic acid 10 mg/L
6
humic acid 20 mg/L 4 2 0 0
5
10
15
20
25
30
Time(min)
Fig. 12. Residual Fe concentration as a function of humic acid concentration.
100
4
90
3.5 3
70
2.5
60 50
DOC removal (%)
2
40
UV254/UV254-0
1.5
30
1
20
0.5
10 0
0 0
5
10
15
20
25
30
Time (min)
Fig. 13. DOC removal and UV254/UV2540.
UV254/UV2540
DOC removal (%)
80
Fig. 14. XRD spectra of solids generated during electrocoagulation 60 min; conditions: pH 7, As(III) 1 mg/L.
Fig. 15. XRD spectra of solids generated during electrocoagulation 30 min; conditions: pH 7, As(III) 1 mg/L.
Fig. 16. XRD spectra of solids generated during electrocoagulation over 30 min; conditions: pH 7, As(III) 1 mg/L, HA 10 mg/L.
Table 1. Comparison between theoretical and experimental iron concentrations as a function of pH and plate number. Plates number
pH
2 3 4 4 4 4
7 7 7 3 5 9
Coefficient of determination (R2) 0.9941 0.9931 0.9974 0.9981 0.9988 0.9991
Mean absolute error 5.2395 3.7432 4.6096 36.2848 2.1671 2.4699
Table 2. Zeta potential as a function of time and additive concentration.
Time( min) 3 5 10 20 30
N o io n, H A 0 0 0 0 0
Zeta( mV)
Time( min)
Mg(mg /L)
Zeta( mV)
Time( min)
Mg(mg /L)
Zeta( mV)
Time( min)
HA(m g/L)
Zeta( mV)
-21.2 -20.3 -2.79 -5.83 2.53
3 5 10 20 30
10 10 10 10 10
-11.9 9.46 10.6 22 27.4
3 5 10 20 30
100 100 100 100 100
22.6 22.2 23.7 22 22.1
3 5 10 20 30
10 10 10 10 10
-32.6 -24.9 -29.4 -13.5 -6.31
Table 3. Zeta potential Time(min) HA(mg/L) Zeta(mV) Time(min) HA(mg/L) Zeta(mV) 3 0 -21.2mV 3 10 -32.6 5 0 -20.3 5 10 -24.9 10 0 -2.79 10 10 -29.4 20 0 -5.83 20 10 -13.5 30 0 2.53 30 10 -6.31
S2213-3437(15)30123-8 http://dx.doi.org/doi:10.1016/j.jece.2015.12.034 JECE 916
To appear in: Received date: Revised date: Accepted date:
27-8-2015 2-12-2015 29-12-2015
Please cite this article as: Han Jo You, Ihn Sup Han, Effects of Dissolved Ions and Natural Organic Matter on Electrocoagulation of As(III) in Groundwater, Journal of Environmental Chemical Engineering http://dx.doi.org/10.1016/j.jece.2015.12.034 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effects of Dissolved Ions and Natural Organic Matter on Electrocoagulation of As(III) in Groundwater Han Jo You, Ihn Sup Han*
Address : School of Environmental Engineering, University of Seoul, Siripdae-gil 163, Dongdaemun-gu, Seoul, Korea
Telephone : +82264905478
Fax : +82264902859
E-mail address: [email protected] (H.J. You)
*Corresponding author. E-mail address : [email protected] (I.S. Han) Telephone : +82264905478
Fax : +82264902859
Highlights The use of an iron electrode resulted in a higher removal efficiency than an aluminum electrode. The fastest removal efficiency was observed at pH 7. When the magnesium concentration was less than 10 mg/L, the arsenic removal was accelerated. The presence of phosphate ions and humic acid decreased the arsenic removal rate.
Abstract Electrocoagulation is an outstanding technique to remove pollutants. When current is applied to electrodes, various amorphous iron hydroxides form complexes. Those complexes with high absorption capacity such as arsenic and heavy metals are removed through the process. In this study, the effects of various electrocoagulation conditions, such as the type of electrode, current, and pH, on the removal efficiency of arsenic were investigated. The removal efficiency varied significantly depending on the type of electrode, as an iron electrode showed superior arsenic removal as compared to an aluminum electrode. As the current increased, the removal rate of arsenic has increased. The validity of the method was examined by calculating the metal elution during electrocoagulation using Faraday’s law and comparing it to the actual elution amount. Notably, amongst the various pH conditions pH 7 generated the fastest removal rate. The effects of dissolved ions on arsenic removal were also examined. When the magnesium concentration was less than 10 mg/L, the initial arsenic removal speed increased. No effects were observed when the concentration of sulfate ions was low (1 mg/L, 10 mg/L). When the concentration of sulfate ions was high (100 mg/L), the arsenic removal rate decreased. In addition, the presence of phosphate ions and humic acid (HA) are reversely correlated with arsenic removal rate.
Keywords: Arsenic, Electrocoagulation, Iron hydroxide, Ion, Humic acid
1. Introduction Arsenic is an artificial pollutant prevalent in leachates in mine areas and agricultural chemicals (e.g., agricultural pesticides and herbicides). Arsenic-containing minerals dissolve into water source and cause natural pollution [1]. Most of the arsenic pollution is natural pollution. Arsenic is abundant in earth’s crust, and thus environmental arsenic pollution is observed worldwide. Arsenic has adverse effects on humans and can cause various skin diseases with long-term exposure; in severe cases, it can cause skin cancer and death [2,3]. World Health Organization (WHO) have established an allowable level of arsenic in drinking water to be 0.01 mg/L, many countries including South Korea abide the standard [3,4]. Globally, numerous studies regarding the removal of arsenic from water have been performed [1]. The main technologies used for arsenic removal include precipitation technologies (coagulation, Fe/Mn oxidation, lime softening), membrane technologies (reverse osmosis, electrodialysis), ion exchange, and adsorption (activated alumina, granular ferric hydroxide, granular titanium dioxide) [5,6]. In particular, electrocoagulation is effective for removal of As(III) [7-9]. Many researchers have found that electrocoagulation is effective for removal of pollutants in water as well as removal of arsenic [10-21]. Equations 1-4 illustrate the reaction that occurs when an iron electrode is used for electrocoagulation [22]: Anode: 4Fe(s) → 4Fe2+(aq) + 8e-
(1)
4Fe2+(aq) + 10H2O(l) + O2(g) → 4Fe(OH)3(s) + 8H+(aq)
(2)
Cathode: 8H+(aq) +8e- → 4H2(g)
(3)
Overall: 4Fe2+(aq) + 10H2O(l) + O2(g) → 4Fe(OH)3(s) + 4H2(g)
(4)
In groundwater, arsenic primarily exists in the trivalent form; many ions and organic matters also exist in groundwater [4]. Metal ions and organic matter are expected to have a large effect on the removal of arsenic from groundwater through electrocoagulation [23]. In this study, effects of magnesium and sulfate as well as phosphate and natural organic matter on the removal of arsenic were investigated. Existing researches have focused only on the efficiency of arsenic removal. Studies about residual iron concentration as secondary pollution are insufficient. High iron ion concentration is not as toxic as arsenic but it has chronic significance on health and environment. The correlation between ion profile of natural water bath and residual iron ion is a key to future research on water resource and environment protection. The study have investigated ion concentration levels within the quality standard of drinking water.
2. Materials and methods 2.1 Materials Samples were prepared using artificial raw water. Basic artificial raw water was made with ultrapure water, As(III), NaCl, HCl, and NaOH. Ultrapure water was the primary source for the artificial raw water and all solutions. To prepare As(III), a stock solution was made using NaAsO2, and the arsenic concentration of the artificial raw water was set to 1 mg/L. NaCl (about 500 μS/cm) was injected at a total concentration of 0.004 M to provide conductivity for the electrocoagulation process. About 0.1 M NaOH and HCl solutions were used to adjust the pH. The pH was determined using a pH meter (238-180 model, Istek, Korea). Only As(III), NaCl, HCl, and NaOH were injected into the raw water. When the effects of ions were examined, each ion species was injected individually, in order to minimize the effects of other ions. Magnesium, phosphate, sulfate, and humic acid stock solutions were made from MgCl2·6H2O, Na2HPO4, Na2O(17.019.0%)+SiO2(35.0-38.0%), Na2SO4, and humic acid, respectively.
2.2 Devices and experimental methods Fig. 1 shows the electrocoagulation and filtering processes used in this study. The size of the reactor was W190 mm * L120 mm * H250 mm. The reactor had a volume of 4 L, and was made of acrylic plates. The size of the electrode plate was W85 mm * L3 mm * H260 mm, and the plates were installed based on a bipolar serial connection (BP-S), where they were fixed at 10 mm intervals so that the wide faces could face each other. Before each experiment, the electrode plate was immersed in 1 M HCl for 10 min, and was then polished using sandpaper. Scale and rust on the iron electrode were removed as mentioned above, and it was then used for the experiment after cleaning with distilled water, which increased the reproducibility of the experiments. A constant current was supplied using a D.C. power supply. The interior of the reactor was uniformly agitated at 60 rpm. During the electrocoagulation process, samples were collected at specified times. The samples were filtered through a 0.45 μm membrane filter (Advantec, mixed cellulose ester) and were subsequently analyzed. Initial arsenic concentration of raw water and accurate sampling time are crucial factors for reducing errors.
2.3 Analytical methods The samples were analyzed using the following machinery and methods. Mg and Fe contents were measured using Inductively Coupled Plasma (ICP, ICPE-9000, Shimadzu, Japan). It was difficult to measure low concentrations of arsenic using ICP alone; therefore, As was analyzed using Inductively Coupled Plasma - Hydride Vapor Generator (ICP-HVG, Shimadzu, Japan). Ferrous iron was analyzed using a UV/VIS spectrophotometer (Evolution 60S, Thermo, USA) based on the 3500-Fe Phenanthroline Method of the Standard Methods [24]. The change in PO43- content was measured using Ion Chromatography (IC, ICS-900, Dionex, USA). To examine the removal efficiency of humic acid, the Dissolved Organic Carbon (DOC) was measured using a Total Organic Carbon Analyzer (TOC Analyzer, TOC-V, Shimadzu, Japan), and the adsorption at 254 nm was measured using a UV/VIS spectrophotometer. A Zetasizer (Nano ZS, Malvern, UK) was used to measure the zeta potential. The electrocoagulation by-
product was precipitated and dried, and was analyzed using X-Ray Diffraction (XRD, D8 advance Sol-X, Bruker, Germany).. In order to reduce sample errors, maintaining standard calibration curves of the equipment played a crucial role to keep consistent and accurate results.
3. Results and discussion 3.1 Comparison of aluminum and iron electrodes Iron or aluminum electrodes are used in most cases for electrocoagulation. Iron electrodes generally are more corrosive than aluminum electrodes; when an iron electrode is used, a red residue is formed. Therefore, for materials that show similar removal efficiencies, aluminum electrodes are preferred [25]. Iron and aluminum electrodes were evaluated in the removal of arsenic. Four electrode plates with the same area were used for both electrodes, and the removal efficiencies were determined at various currents. A distinct difference between the electrodes was observed, as shown in Fig. 2. With the iron electrode, the removal efficiency was 99% within 10 min at 0.2 A; at 0.1 A, a removal efficiency of 99% was achieved 15 min. At 0.05 A, the removal efficiency was higher than 99% within 25 min. However, with the aluminum electrode, the arsenic removal efficiency was less than 50% until 30 min at 0.2 A and 0.1 A. As the current increased, the elution of the metal electrode increased, and thus the formation of metal hydroxides increased. Presumably, the fast-forming metal hydroxide aids in the quick removal of arsenic. However, the aluminum electrode showed significantly lower arsenic removal than the iron electrode. This indicated that iron hydroxide binds better to arsenic than aluminum hydroxide [9]. In subsequent experiments, 0.1 A current was used instead of 0.2 A, because the fast removal of arsenic was not distinctly different in the presence of other ions.
3.2 Elution of iron In electrocoagulation, the amount of metal ions dissolved in water from an electrode plate is proportional to the reaction time and current. The current was fixed at 0.1 A at pH 7, and the electrical conductivity was adjusted using NaCl. First, the elution was compared by varying the number of plates (2, 3, and 4 plates). As the number of plates increased, the concentration of iron also increased. This was compared to the theoretical concentration using Faraday’s law [3,26]. When 2 iron plates were used, the coefficient of determination (R2) between the theoretical and experimental values was very high (0.9941), as shown in Fig. 3. The absolute average error was also very small (5.2395).
However, when 3 and 4 plates were used, the concentration of iron increased continuously. The results were compared by including a factor for the number of plates in Faraday’s law, as shown in equation 5: 1
(5)
where W is the mass of the dissolved metal (g), I is the applied current (A), t is the treatment time of the
electrocoagulation process (s), M is the molar mass of the anode metal (g/mol), z is the valence number of ions of the substance (zAl = 3, zFe = 2), F is Faraday's constant (96,485 C/mol), and n is the number of plates. The coefficients of determination (R2) between the theoretical and experimental values when the plate numbers were 3 and 4 were 0.9931 and 0.9974, respectively, as summarized in Table 1. The absolute average errors were small (3.7432 and 4.6096, respectively). As a result, when more than 2 plates were used, the metal elution was proportional to 1 less than the number of plates (n). When the experiment was performed at pH values of 5, 7, and 9 using 4 plates, the absolute average errors were within 5%. Therefore, the use of Faraday’s law was appropriate. However, at pH 3, the absolute average error was 36.2848%. The experimental value was significantly larger than the theoretical value because the dissolution rate of the positive electrode was accelerated under acidic conditions. At pH values less than 4, hydrogen evolution occurs, and it is called hydrogen evolution-type corrosion. At pH values between 4 and 10, the corrosion rate is nearly constant [27]. At approximately pH 7, the protective oxide film on the electrode surface is not stable, and thus the iron elution at the electrode is high [28]. At pH 3, the rate at which the pH increases is slow, and thus more elution of iron occurs until the pH increases above a certain value. Therefore, Faraday’s law could not be applied at pH 3.
3.3 Effect of initial pH on removal efficiency In the following experiment, 4 iron plates were used, and the effect of pH on arsenic removal was examined by adjusting the pH (3, 5, 7, and 9) using NaOH and HCl. At pH 7, the arsenic content reached 0.01 mg/L (i.e., the standard for drinking water) in the shortest amount of time, as shown in Fig. 4. During the initial 8 min of the 30-minute electrocoagulation process, the highest removal efficiency was observed at pH 5. Due to the fast oxidation of iron, arsenic was removed quickly. However, after 10 min, the best arsenic removal efficiency was observed at pH 7. At pH 7, after 10 min, fast-grown flocs were filtered by a membrane filter, and a higher removal efficiency was observed. At pH 9, the removal efficiency was lower than that at an initial pH of 5, but was similar to that at pH 7. After 5 min, the removal efficiency significantly decreased compared to those at pH values of 7 and 5. Although the removal rate decreased, the water quality standard of arsenic (0.01 mg/L) could be satisfied within 30 min at pH 9. However, at pH 3, the arsenic content could not reach 0.01 mg/L within 30 min. Fig. 5 shows the concentration of iron that remained in water after electrocoagulation and membrane filtering. At pH 7 and 9, when the arsenic concentration decreased below 0.01 mg/L, the iron concentration also decreased below 0.3 mg/L (i.e., the water quality standard of drinking water). During the initial stages, iron mostly existed as ferric iron, and iron hydroxide remained as small particles. The amount of remaining ferrous iron was much less than that of ferric iron. However, at pH 3 and 5, a high concentration of iron (more than 0.3 mg/L) was observed. Most of the remaining iron at pH 3 and 5 was ferrous iron, and it could not be oxidized to ferric iron and remained in the water. The concentration of dissolved iron was likely high because iron hydroxide could not form because the dissolution of iron hydroxide is high at low pH. Moreover, the iron dissolution increased as the pH decreased. Removal of arsenic absorbed by iron hydroxide through membrane filtration decreased at pH 5. Fig. 4 shows arsenic concentration fall in pH7 after 10min compared to that of pH5.
3.4 Effects of metal ions and humic acid Next, we examined the effects of dissolved ions on the removal of arsenic from groundwater via electrocoagulation. First, to investigate the effects of ion concentration, different concentrations of selected ions were injected at a current of 0.1 A, and the pH was adjusted to 7.
3.4.1. Effects of magnesium First, the effects of dissolved magnesium ions on the removal of arsenic from groundwater via electrocoagulation were evaluated. A concentration of 0.1 mg/L of magnesium had almost no effect on the arsenic removal (data not shown). When the concentrations of magnesium were 1 mg/L and 10 mg/L, the arsenic removal rate increased; this trend was more distinct when the concentration of magnesium was high (10 mg/L). However, the presence of 100 mg/L of magnesium decreased the arsenic removal rate. As shown in Fig. 7, more than 0.3 mg/L (i.e., water quality standard of drinking water) of iron remained when 10 mg/L of magnesium was injected, and the amount of iron continuously increased as time passed. In particular, when 100 mg/L of magnesium was added, the concentration of remaining iron was compared to the total amount of iron dissolved in water that was calculated using Faraday’s law. The concentration of iron remaining in water after 3 min was about 44% of the total amount of iron, and it was about 55% at 30 min, where it steadily increased as time passed. The remaining iron was identified as ferrous iron, while unoxidized iron was present in the filtered water. Because magnesium combines with hydroxide ions to produce Mg(OH)2 [29,30] (Eq. 7), the production of iron hydroxide decreased. Thus, ferrous iron in water could not be oxidized and remain in the water. The decrease in iron hydroxide content reduced the arsenic removal rate.
MgCl2 + 2OH- → Mg(OH)2 + Cl2
(7)
The change in magnesium content could were also examined using ICP. The decrease in magnesium was about 10% at all concentrations excluding 0.1 mg/L.
The solubility product of Mg(OH)2 ⇄ Mg2+ + 2OH- (Ksp at 25°C) is 9×10-12 [31]. It is a solid and it dissociates in water. Thus, more OH- was deficient, and this resulted in the decrease in iron hydroxide. Table 2 summarizes the zeta potential measurements. With 100 mg/L of magnesium, the zeta potential was positive at 3 min. Negatively charged colloids or anions were quickly removed as they approached 0 mV due to the dissolution of iron (i.e., cation). However, the arsenic removal was not efficient because of the reversed potential, due to the high concentration of magnesium.
Appropriate amounts of magnesium had a positive effect on arsenic removal, but high concentrations of magnesium decreased the arsenic removal rate due to the increase in the zeta potential and the inhibition of
coagulant formation. To prevent this, large amounts of magnesium should be removed first. Additionally, the pH should be increased, and the time should be adjusted to reduce the iron elution.
3.4.2. Effects of sulfate ions The effects of sulfate ions on arsenic removal were evaluated. Changes in the arsenic removal were examined by injecting 0.1, 1, 10, and 100 mg/L solutions of sulfate ions at pH 7. The results obtained in the presence of 0.1 mg/L sulfate ions are not shown in Fig. 8. With 1 and 10 mg/L sulfate ions, the results were similar to that without sulfate. However, at 100 mg/L, the arsenic removal rate decreased. In previous studies, it was reported that sulfate ions had almost no effect on arsenic removal [8,23]. In the present study, 10 mg/L of sulfate also had no effect on the arsenic removal rate. However, at high concentrations of sulfate (100 mg/L), the arsenic removal rate decreased. SO42- probably competed with arsenic for the adsorption site of iron hydroxide [32]. Also, considering that the concentration of iron initially increased and then decreased when the sulfate concentration was 100 mg/L (Fig. 8), the oxidation of iron was probably inhibited.
3.4.3 Effects of phosphate ions Phosphate ions have been reported to inhibit removal of arsenic. Arsenic and phosphorus are located in the same group of the periodic table, and have the same tetrahedral structure. Thus, they are removed by adsorption on the same adsorption site of iron hydroxide [8,33,34]. Phosphate ions are less abundant in groundwater than other ions. In this experiment, 0.1, 1, and 10 mg/L of phosphate ions were added, and the removal efficiency of arsenic as well as the concentration of dissolved iron were examined. As shown in Fig. 9, the arsenic removal rate was affected by the presence of 0.1 mg/L phosphate, unlike that with other ions. At about 8 min, 0 and 0.1 mg/L showed a C/C0 difference of about 0.2. The times required to reach the drinking water quality standard were not significantly different. However, with 0.1 mg/L phosphate, it took more time for the concentration of iron to decrease below the drinking water quality standard. As the concentration of phosphate ions increased, the removal rate decreased. When 10 mg/L of phosphate ions were injected, the arsenic removal efficiency was 70% within 30 min, and the drinking water quality standard could not be satisfied. With 10 mg/L of phosphate ions, the arsenic removal efficiency depending on the initial pH was examined by increasing the electrocoagulation process time to 60 min. The concentration of phosphate ions was measured using ion chromatography; phosphate ions were not detected in the sample collected at 5 min at 1 mg/L. The removal of phosphate ions was faster than that of arsenic. Furthermore, when the concentration of iron decreased below the standard value, it was nearly consistent until the water quality standard for arsenic was reached. This indicated that arsenic and phosphate ions have a competitive relationship, but phosphate ions are quickly removed because they react with iron faster than arsenic, and that the removal of arsenic is relatively more difficult.
Fe3+ + PO43- → FePO4
(8)
Phosphate is removed by the above mechanism (Eq. 8), and it could also be removed by adsorption on iron hydroxide [35,36]. Due to this removal mechanism, OH- produced during the electrocoagulation process remained in the water and did not form iron hydroxide, which increased the pH. This increase was compared
to that observed in other experiments. With 10 mg/L of magnesium (Fig. 6) and 0 mg/L of phosphate (Fig. 7), the pH values after 15 min were 6.67 and 7.18, respectively, but for 10 mg/L of phosphate, the pH increased to 9.52.
When 10 mg/L of phosphate ions were dissolved in water, the arsenic removal efficiency was low. Therefore, the process time was increased to 60 min. However, after 60 min, the removal efficiency was only about 90%, and the drinking water quality standard could not be satisfied.
3.4.4 Effects of humic acid The effects of humic acid on the removal of arsenic from groundwater were also evaluated. Humic acid concentrations of 1 mg/L, 5 mg/L, 10 mg/L, and 20 mg/L were assessed. Fig. 11 shows the removal efficiency of arsenic in the presence of humic acid. There was almost no difference in the arsenic removal rate or iron concentration upon addition of 1 mg/L humic acid as compared to that without humic acid. When the concentrations of humic acid were 5 mg/L, 10 mg/L, and 20 mg/L, the arsenic removal rate decreased as the concentration increased, and a significant amount of time was required to reach a concentration below the drinking water quality standard. The target water quality was obtained at 20 min for 10 mg/L, and at 25 min for 20 mg/L. Fig. 12 shows the concentration of remaining iron. The iron concentration increased as the humic acid concentration increased. This pattern was similar to that of other ions, but there was a slight difference. While the concentration of remaining iron ions increased during the initial stage of the process, the decrease in arsenic content was smaller than the case in which the inhibition of arsenic removal by ions was observed. When the concentration of remaining iron began to decrease, the arsenic and iron concentrations abruptly decreased. In particular, the difference was large when the ion concentration was high.
These results were attributed to the sweep coagulation of humic acid. Humic acid is comprised of hydrophobic and hydrophilic materials. In particular, the hydrophobic compounds constitute about 90% of the total material. In general, hydrophobic compounds are more easily coagulated [37]. Thus, the coagulation and formation of iron hydroxide flocs are improved; as a result, sweep coagulation occurs due to the decrease in the zeta potential of the floating colloidal material and the promotion of colloidal particle removal by trapping [38]. The removal of humic acid was measured using the adsorption at 254 nm and DOC. In previous study, it was mentioned that the increase in the adsorption at 254 nm during electrocoagulation using an iron electrode was due to production of colored iron hydroxide particles [39]. In this study, the UV adsorption at 254 nm also increased with the addition of 10 mg/L of humic acid. The adsorption at 254 nm increased to 3.6 times the initial value, then decreased to 0.05 after 15 min; DOC revealed a removal efficiency of 83.5%. This value was continuously maintained until 30 min. In addition, as mentioned previously, the concentrations of arsenic and iron decreased abruptly at 15 min, indicating that humic acid and arsenic were removed simultaneously.
Table 3 compares the cases in which there was humic acid in water when there was no humic acid. Organic acids such as humic acid have a negative charge in water, which usually inhibits coagulation and adsorption. In this experiment, humic acid exhibited a negative charge and lowered the coagulation and adsorption of arsenic [39].
3.5 XRD To confirm the formation of iron hydroxide, the particles produced after the electrocoagulation process were collected and dried, and were analyzed using X-ray Diffraction (XRD). Fig. 14 and Fig. 15 show the XRD of the products obtained after 60- and 30-minute electrocoagulation processes. Respectively, at pH 7 when the arsenic concentration was 1 mg/L. Iron existed as lepidocrocite (γ-FeO(OH)) and magnetite (Fe3O4) (Fig. 14). However, only lepidocrocite was observed after 30 min (Fig. 15), indicating that the type of iron that formed varied as time passed. Magnetite is an oxidized form of iron, and its formation over time is consistent with the electrocoagulation mechanism mentioned previously [8,40]. Fig. 16 shows the results of a 30-minute electrocoagulation process at pH 7 when 1 mg/L As(III) and 10 mg/L humic acid were present, only lepidocrocite was observed in this process. Lepidocrocite formation was observed under several sets of conditions (Figs. 14, 15, and 16), but the peaks intensity in the XRD spectra became sharper and clearer as time passed. In addition, when arsenic or humic acid were present, the peak intensity became weak.
Through XRD analysis, the electrocoagulation mechanism was determined to proceed as shown in equations 9-12.
Early stage: Fe + 3(OH) → Fe(OH)3 Fe(OH)3(s) → FeO(OH)(s) + H2O(l) (lepidocrocite)
(9) (10)
Later stage: 4Fe + 12(OH) → 4Fe(OH)3
(11)
4Fe(OH)3(s) → Fe(OH)2(s) + Fe3O4(s) + H2O(l) (magnetite) (12)
During electrocoagulation, arsenic is likely removed via adsorption and co-precipitation by Fe(OH)3 and lepidocrocite [26]; during the late stages of electrocoagulation, arsenic is removed by the formation of magnetite [40].
4. Conclusions In this study, the effects of various ions and humic acid on the removal of As(III) via electrocoagulation were investigated. The use of an iron electrode resulted in a higher removal efficiency than an aluminum electrode. The time required to reach the drinking water quality standard varied depending on the pH. The fastest removal efficiency was observed at pH 7, followed by pH 5, pH 9, and pH 3. Magnesium increased the arsenic removal rate. However, high magnesium concentrations inhibited arsenic removal, and abruptly increased the concentration of dissolved iron. A high concentration of sulfate or phosphate ions decreased the arsenic removal rate, and resulted in a substantial increase in the pH. In addition, humic acid also decreased the arsenic removal rate. XRD analysis indicated that iron was oxidized to lepidocrocite initially and subsequently oxidized to magnetite during the later stages of the process. These iron hydroxide and iron oxide species aided in the effective removal of arsenic.
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Figures and figure captions
Fig. 1. Electrocoagulation and membrane process.
100 0.2A Fe 0.1A Fe 0.05A Fe 0.1A Al 0.2A Al
Arsenic removal (%)
80 60 40 20 0 0
5
10
15
20
25
30
Time(min)
Fig. 2. Arsenic removal as a function of current on iron and aluminum plates.
Experimental concentration of Fe (mg/L)
7 6 5 4 3 2 1 0 0
1
2
3
4
5
6
7
Theoretical concentration of Fe (mg/L)
Fig. 3. Comparison between theoretical and experimental iron concentrations using 2 iron plates at pH 7.
1 pH 3 0.8
pH 5 pH 7
0.6 C/C0
pH 9
0.4 0.2 0 0
5
10
15
20
25
30
Time(min)
Fig. 4. Arsenic removal as a function of initial pH after electrocoagulation and membrane process at 0.1 A.
10
Fe concentration(mg/L)
pH 3 8
pH 5 pH 7
6
pH 9
4 2 0 0
5
10
15
20
25
30
Time(min)
Fig. 5. Residual Fe concentration as a function of initial pH after electrocoagulation and membrane process.
1 Magnesium ion 0 mg/L
0.8
Magnesium ion 1 mg/L 0.6 C/C0
Magnesium ion 10 mg/L Magnesium ion 100 mg/L
0.4 0.2 0 0
5
10
15
20
25
30
Time(min)
Fig. 6. Effects of magnesium concentration on arsenic removal.
Fe Concentration(mg/L)
10.000 8.000 6.000 Magnesium ion 0 mg/L Magnesium ion 1 mg/L
4.000
Magnesium ion 10 mg/L Magnesium ion 100 mg/L
2.000 0.000 0
5
10
15
20
25
30
Time(min)
Fig. 7. Residual Fe concentration as a function of magnesium ion concentration.
1 0.8 Sulfate 0 mg/L 0.6 C/C0
Sulfate 1 mg/L Sulfate 10 mg/L
0.4
Sulfate 100 mg/L
0.2 0 0
5
10
15
20
25
30
Time(min)
Fig. 8. Effect of sulfate concentration on arsenic removal.
1
phosphate 0 mg/L phosphate 0.1 mg/L phophpate 1 mg/L phosphate 10 mg/L
0.8
C/C0
0.6 0.4 0.2 0 0
5
10
15
20
25
30
Time(min)
Fig. 9. Effect of phosphate concentration on arsenic removal.
12
Fe Concentrion(mg/L)
10 8 6 phosphate 0 mg/L phosphate 0.1 mg/L phophpate 1 mg/L phosphate 10 mg/L
4 2 0 0
5
10
15
20
25
30
Time(min)
Fig. 10. Residual Fe concentration as a function of phosphate concentration.
1 humic acid 0 mg/L
0.8
humic acid 5 mg/L 0.6 C/C0
humic acid 10 mg/L humic acid 20 mg/L
0.4 0.2 0 0
5
10
15
20
25
30
Time(min)
Fig. 11. Effect of humic acid concentration on arsenic removal.
12
Fe Concentration(mg/L)
10 humic acid 0 mg/L 8
humic acid 5 mg/L humic acid 10 mg/L
6
humic acid 20 mg/L 4 2 0 0
5
10
15
20
25
30
Time(min)
Fig. 12. Residual Fe concentration as a function of humic acid concentration.
100
4
90
3.5 3
70
2.5
60 50
DOC removal (%)
2
40
UV254/UV254-0
1.5
30
1
20
0.5
10 0
0 0
5
10
15
20
25
30
Time (min)
Fig. 13. DOC removal and UV254/UV2540.
UV254/UV2540
DOC removal (%)
80
Fig. 14. XRD spectra of solids generated during electrocoagulation 60 min; conditions: pH 7, As(III) 1 mg/L.
Fig. 15. XRD spectra of solids generated during electrocoagulation 30 min; conditions: pH 7, As(III) 1 mg/L.
Fig. 16. XRD spectra of solids generated during electrocoagulation over 30 min; conditions: pH 7, As(III) 1 mg/L, HA 10 mg/L.
Table 1. Comparison between theoretical and experimental iron concentrations as a function of pH and plate number. Plates number
pH
2 3 4 4 4 4
7 7 7 3 5 9
Coefficient of determination (R2) 0.9941 0.9931 0.9974 0.9981 0.9988 0.9991
Mean absolute error 5.2395 3.7432 4.6096 36.2848 2.1671 2.4699
Table 2. Zeta potential as a function of time and additive concentration.
Time( min) 3 5 10 20 30
N o io n, H A 0 0 0 0 0
Zeta( mV)
Time( min)
Mg(mg /L)
Zeta( mV)
Time( min)
Mg(mg /L)
Zeta( mV)
Time( min)
HA(m g/L)
Zeta( mV)
-21.2 -20.3 -2.79 -5.83 2.53
3 5 10 20 30
10 10 10 10 10
-11.9 9.46 10.6 22 27.4
3 5 10 20 30
100 100 100 100 100
22.6 22.2 23.7 22 22.1
3 5 10 20 30
10 10 10 10 10
-32.6 -24.9 -29.4 -13.5 -6.31
Table 3. Zeta potential Time(min) HA(mg/L) Zeta(mV) Time(min) HA(mg/L) Zeta(mV) 3 0 -21.2mV 3 10 -32.6 5 0 -20.3 5 10 -24.9 10 0 -2.79 10 10 -29.4 20 0 -5.83 20 10 -13.5 30 0 2.53 30 10 -6.31